The present invention relates to a semiconductor light emitting device which is applicable to, e.g., a white light emitting diode and to a method for fabricating the same.
A gallium nitride-based (GaN-based) group III-V nitride semiconductor (InGaAlN) in which GaN has a large forbidden bandwidth of, e.g., 3.4 eV at a room temperature is a material which can implement a light emitting device capable of a high output in the blue or green wavelength band or even in the ultraviolet wavelength band. The GaN-based semiconductor has already been commercialized as a blue/green light emitting diode in various display panels, large-scale display devices, and traffic signals.
On the other hand, a white light emitting diode which provides white light by exciting a YAG fluorescent material with emission light from a blue light emitting diode has also been commercialized already for various applications including the back light of a liquid crystal display panel.
If the white light emitting diode can be enhanced in brightness and light emitting efficiency, a semiconductor illuminating device as a replacement for currently prevailing fluorescent and incandescent lamps can be provided. Accordingly, white light emitting diodes for illumination are predicted to create an extremely large market in the future.
For illumination purposes, the improvement of the manner in which color appears when a white light emitting diode is used for illumination, i.e., a color rendering property is important in addition to the enhancement of brightness and the light emitting efficiency.
Since each of the white light emitting diodes that have been commercialized heretofore has used a method which excites a YAG (Yttrium Aluminum Garnet) fluorescent material with emission light at about 470 nm from a blue light emitting diode and thereby obtains yellow emission light (see, e.g., S. Nakamura et al., “The Blue Laser Diode” Springer-Verlag Berlin Heidelberg New York: See p. 216), the problem has been encountered that the amount of red emission light in an emission spectrum is small and the color rendering property is consequently inferior to that of light from a fluorescent lamp and light from an incandescent lamp.
At present, there is no red fluorescent material that exhibits a sufficiently high excitation efficiency upon excitation caused by blue emission light. To improve the color rendering property, therefore, it is particularly necessary to enhance the brightness of a red light emitting fluorescent material.
Referring to FIG. 36, a description will be given herein below to a structure of a conventional white light emitting diode which provides white light by mixing blue light from a GaN-based blue light emitting diode with yellow light which is outputted through the excitation of a YAG fluorescent material with the blue light and to the light emission characteristics thereof.
As shown in FIG. 36, the white light emitting diode is composed of a blue light emitting diode 300 for outputting blue light at a wavelength of 470 nm which has been mounted on a package 310 to have upper and side surfaces thereof covered with an insulating material 320 containing a YAG fluorescent material.
A method for fabricating the blue light emitting diode 300 will be described, in which an n-type GaN layer 302, an active layer 303 made of InGaN, and a p-type GaN layer 304 are formed successively on a substrate 301 made of sapphire by, e.g., MOCVD (Metal Organic Chemical Vapor Deposition),
Next, dry etching using, e.g., a chlorine gas is performed with respect to the n-type GaN layer 304 and the active layer 303, thereby selectively exposing a part of the n-type GaN layer 302. Subsequently, an n-side electrode 305 composed of a multilayer film of titanium and gold is formed on the exposed portion of the n-type GaN layer 302. On the other hand, a p-side transparent electrode 306 composed of a multilayer film of nickel and gold and having a reduced thickness of 10 nm or less for the transmission of light emitted from the active layer 303 is formed on the p-type GaN layer 304.
Next, a pad electrode 307 made of gold is formed selectively on the p-side transparent electrode 306, whereby a majority of the blue light emitted from the active layer 303 passes through the p-side transparent electrode 306.
Next, the blue light emitting diodes 300 in the form of a wafer is divided into square chips each having, e.g., 350-μm sides. Each of the chips is mounted at a specified position on the package 310 and then wire bonding is performed with respect to the n-side electrode and the pad electrode 307. Subsequently, the insulting material 320 containing the YAG fluorescent material is coated or applied dropwise to cover the blue light emitting diode 300 and then hardened.
The light emission characteristics of the white light emitting diode thus obtained are as shown in FIG. 37 so that white light which is a mixture of blue emission light 300A from the blue light emitting diode 300 and yellow emission light 320A from the YAG fluorescent material 320 is emitted.
However, the conventional white light emitting diode has the problem of a poor white color rendering property, as shown in FIG. 37, due to the small amount of the red emission light component in a spectrum obtained from the blue light emitting diode 300.